Zipdy: Free Zip Code Distance Calculator

Zipdy is a program for calculating the distance between two zip codes and
finding all the records in a RDBMS with a zip code with x miles of another
zip code. Currently, RDBMS support exists for postgreSQL. Additional future support for other databases will be
added in future releases of zipdy.

Zipdy is public domain software. Zipdy was written in C but is also
available in PHP and perl. It runs with data from the US Census
Gazetteer
program. The original Gazetteer data is available here.
The data is from the 2000 census.

The distance calculations are made by doing a great circle
calculation on longitude and latitude points provided by the
census data. The result is returned in standard miles. If you
need an explanation of what a great circle distance calculation
is, click here.

Main Features of Zipdy:

Calculate the distance between two zip codes

Query against a RDBMS for all address records zip codes with in X miles of a zip code

Note: Datafiles where not updated for version
2.5.0. To upgrade, you only need to download the source archive.

If you'd like to place zipdy on your web site just to calculate
the distance between two zip codes, you'll need to acquire GDBM
support for your platform if you don't have it. Then you'll need
to download these files: